Electrochemical behaviour of alloy CoW0.013C0.001 in acidic sulphate solutions

Elucidation of the electrochemical behaviour of the CoWxCy binder alloy is a basic step in understanding the corrosion performance of hard metals of the WC-Co type. The anodic oxidation mechanism of the alloy CoW0.013C0.001 has been studied in 0.1 M H2SO4 by linear sweep and cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. The morphology, composition, and structure of the reaction products were studied by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction, respectively. In this paper, the main features of the pseudopassivating behaviour are described and a simple model for the reaction kinetics is suggested. The development of corrosion layers with different inhibiting effectiveness is reported and the relationship between the features of these layers and the evolution of the surface composition is highlighted. The formation of a hydrated mixed oxide (CO3W7O24 center dot 19H(2)O) is revealed, which provides evidence for the complexity of the reactions involved. A comparison of the corrosion product morphologies and surface structures suggests a correlation between the orientation of the surface grains and the type and stability of the inhibiting layer.